`RepairFailingRequirement` Strategy

[nrfulton]

It would be helpful to have more types of strategies to show to new developers. We can start with adding a RepairFailingRequirement strategy and documenting it in a Contributor Guide for strategies.

• Add a strategy that repairs based upon failing requirements.
• Add (and use) a Context.fork() for this strategy.

[HendrikStrobelt]

partially implemented by #65